1. Field of the Invention
The present invention relates broadly to methods of signal modulation and demodulation, particularly methods of quadrature amplitude modulation, for facilitating data transmission and other communication applications. More particularly, the present invention concerns a method of quadrature amplitude modulation involving encoding phase differentially and amplitude absolutely to achieve a high data rate and spectral efficiency in data transmission and other communication applications, and also for allowing amplitude scaling to facilitate data recovery; amplitude scale tracking; 2N power carrier recovery; incoherent demodulation; coherent demodulation; multipath equalization; and demodulation filtering.
2. Description of the Prior Art
It is often desirable to maximize data rate and spectral efficiency in communication applications. Quadrature amplitude modulation (QAM), for example, is one means of doing so. QAM is a well-known modulation method combining amplitude and phase modulation in which two sinusoidal carriers, one exactly 90° or ¼ cycle out of phase with respect to the other, are used to transmit data over a single channel. Because the carriers differ by a 90° phase shift, they are orthogonal and can be modulated independently, transmitted over the same frequency band, and separated during demodulation at the receiver. Thus, for a given available bandwidth, QAM enables higher data transmission rates than other prior art modulation methods. QAM and its derivatives are used in mobile radio and satellite communication systems and other wireless and cable data transmission applications.
Prior art QAM methods, however, suffer from a number of problems and disadvantages, including, for example, that they typically encode both amplitude and phase absolutely and do not allow for continuous referencing. Furthermore, prior art methods typically do not satisfactorily address changes in signal strength or rapid and severe scale variations, and therefore do not facilitate successful signal demodulation and data retrieval under such conditions. Additionally, prior art methods typically do not satisfactorily allow for incoherent carrier recovery when coherent recovery is impossible due to signal degradation. Additionally, prior art methods typically do not satisfactorily address multipath equalization under conditions of severe frequency dependent fading.
It should be noted that while references to so-called “differential” QAM methods can be found in prior art literature, these methods are related to bit assignment and typically continue to require coherent demodulation or involve amplitude ratioing which sacrifices bit-error-rate (BER) performance.
Due to the above-identified and other problems and disadvantages in the art, a need exists for an improved method of signal modulation and demodulation for maximizing data rate and spectral efficiency in data transmission and other communication applications